Bouldnor formation
The Bouldnor Formation is a geological formation in the Hampshire Basin in southern England . It is the youngest formation of the Solent Group and was deposited in the Upper Eocene and Lower Oligocene . The Grande Coupure documents its fauna content .
Type locality and occurrence
The Bouldnor formation was named after the town of Bouldnor east of Yarmouth ( Isle of Wight ) . Along the coastline between Yarmouth and Hamstead in the north-west of the island, the entire formation, which was folded here into an east-south-east trending, open syncline - the Bouldnor Syncline - is very nicely exposed.
However, the type locality (stratotype) of the formation is Whitecliff Bay on the east coast of the Isle of Wight .
history
The Bouldnor Formation been scientifically established in 1985 by A. B. Insole & Daly and its layer members ( engl. Member ) are defined. A first description of the paleogene of the Isle of Wight goes back to E. Forbes in 1853. He was followed by HJO White in 1921.
stratigraphy
The Bouldnor Formation is the final formation of the Solent Group in the Hampshire Basin , after which the basin fell dry. The thickness of the formation varies between 45 and 115 meters. After a long hiatus , enveloping sediments of the Pleistocene and Holocene laid discordant over the formation. At its base, the Bouldnor Formation concordantly follows dry cracks in the uppermost Bembridge Limestone of the Bembridge Limestone Formation , a freshwater limestone .
The Bouldnor Formation consists mainly of clays and subordinate sands , which were deposited along a coastal plain in various facies areas close to the beach ( lagoon , lacustrine / palustrine and fluvial ) - recognizable by the fauna communities it contains from the freshwater, brackish water and marine areas. Marine conditions such as the Bembridge Oyster Bed , Nematura Bed and the upper Cranmore-Member were seldom realized.
The formation contains a diversified and well-preserved fauna community consisting of mollusks , vertebrates (especially mammals ), charophytes, and vascular plants. The non-marine sections are characterized by the gastropods Australorbis , Lymnaea and Viviparus as well as the ostracods Candona , Cypridopsis and Moenocypris . In its middle section (Hamstead member) the faunal section of the Grande Coupure can be seen, which correlates with the negative oxygen isotope anomaly Oi-1 of the lowest Oligocene.
The Bouldnor formation is stratigraphically divided into the following members (from hanging to lying):
- Cranmore member
- Hamstead member
- Bembridge Marls Member
Bembridge Marls Member
The basal, about 20 to 23 meters (sometimes up to 35 meters) mighty Bembridge-Marls-Member is made up of blue-gray to green-gray, finely layered clays and marls . It contains numerous horizons with layers of molluscs. The clays show varveal stratification . The member lies concordantly over the Bembridge Limestone Formation, the final horizon of which is traversed by dry cracks . The member correlates to a large extent magnetostratigraphically with the upper section of Chron C 13r , and biostratigraphically with the calcareous nanoplankton zone NP21 . It therefore belongs to the uppermost Priabonium and should be between 34.0 and 33.75 million years old.
The Bembridge-Marls member was mainly sedimented in fresh and brackish water (indicated by Cirripedia and the gastropod Tarebia ). The lower section comes from the estuarine area and the upper section is of fluvial origin , recognizable by the prosobranch taxon Viviparus . However, horizons such as the Bembridge Oyster Bed 1.5 meters above the base and above a calcareous layer with corbiculid bivalvia and nucula also suggest relatively short-term marine incursions.
The fossils contained are the freshwater taxa Lymnaea and Unio and the marine species Melanopsis , Meretrix and Ostrea . Of fish present are Amia sp. and other indefinite Amiidae . The Bembridge Insect Bed at the base is a sand-lime layer that contains a very rich insect fauna and many leaf prints . The layer represents a fossil deposit with very good preservation. Coleoptera , Diptera , Hymenoptera and arthropods such as Aeschnophlebia andeasi , Oligoaeschna anglica and Vectaraneus yulei were found .
Plant finds in the Bembridge-Marls-Member are palm pollen and the fern Acrostichum .
Of mammal remains were found Anoplotherium latipes and Anoplotherium commune , Bransatoglis bahloi , Choeropotamus parisiensis , Ectropomys exiguus , sp Gesneropithex. , Glamys devoogdi , Haplomeryx zitteli , Heterohyus , Microchoerus edwardsi , Palaeotherium medium , Paroxacron sp. , Peratherium , Plagiolophus major , Plagiolophus minor , Saturninia gracilis , Suevosciurus ehingensis , Tarnomys schmidtkittleri , Theridomys bonduelli and Treposciurus .
Hamstead member
The Hamstead Member , 20 to 70 meters high , is divided into two parts by the Nematura Bed (named after the gastropod Nematura parvula ).
The 10 meter thick Lower Hamstead member follows concordantly with the basal, 40 centimeter thick Black Band - an olive-black, organic silt layer - on the Bembridge-Marls member. The Black tape was deposited in fresh water and leads to his Bais Krustenkalkknollen (Engl. Calcrete nodules ) and Wurzelrhizome . About 4 meters thick, green-gray alternating layers of clay and silt lie on top. Then 3 meters of blue to brown, fine-layered clays with shell layers in the central area of the sequence sedimented. The clays are covered discordantly by up to one meter thick, pale, blue-gray, clayey sands, revealing the ball-and-pillow structure , winding layers and convolute bedding . This very unstable location is known as a log bed , as it contains tree trunks up to 5 meters long. In the log bed, which clearly belongs to the freshwater area, with alluvial seeds of Potamogeton and Stratiodes as well as mono- and dicotyledonous leaves, there are also the last fauna elements ( MP20 ) in front of the Grande Coupure.
The Lower Hamstead Member ends after the log bed and a significant gap in the shift with the discordant, almost 1 meter thick Nematura bed . The Nematura bed consists of characteristic, chocolate-brown ribs in alternating clays and sands. It is brackish and contains remanufactured wood. In fossils, it contains the mollusks Nematura (now Stenothyra ) and Polymesoda, the ostracode Hemicyprideis and marine dinoflagellates. Evidence of an eroded paleo-soil was found in its basal shell position . A period of 350,000 years is estimated for the shift gap.
From the lower Hamstead-members where taxa originate Amphidozotherium cayluxi , exile Amphiperaterium , Anoplotherium commune , Anoplotherium latipes , Bransatoglis planus , Butselia biveri , Cryptopithecus , Eotalpa anglica , Glamys fordi , Palaeotherium curtum , Palaeotherium muehlbergi , paradoxonycteris tobieni , Pseudoltinomys cuvieri , sp Ronzotherium. , Stehlinia minor , Suevosciurus ehingensis , Suevosciurus fraasi , Theridomys bonduelli and Xiphodon gracilis .
Conifers are now appearing on plants , for example Quasisequoia couttsiae with the pollen Inaperturopollenites magnus .
The Upper Hamstead Member , which is up to 60 meters thick, starts again with 3 meters thick, green-gray clay-silt alternating layers with decalcified Palymesoda shells. About 10 meters above the base follows the Eomys bed and shortly afterwards the White Band , also with polymesoda bowls. After the interposed crocodile bed , the member ends with 8 meters of turquoise, plastic clay that is speckled orange-red. They carry the occasional brown, laminated pitch and some clam horizons. The plastic clays show armor straps . Also worth mentioning is the white lily bed in the upper third.
The remains of mammals were also found in the Upper Hamstead Member, the ensemble of which clearly shows the faunistic effects of the Grande Coupure . The Grande Coupure must therefore have taken place between the Lower and Upper Hamstead Member, the first elements of the fauna after the Grande Coupure appear 4 meters after the start of the Upper Hamstead Member.
The Upper Hamstead Member contains the following taxa: Amphicynodon sp. , Amphiperatherium exile , Amphiperaterium minutum , Asteneofiber , Atavocricetodon atavus , Bothriodon velaunus , Butseloglis micio , Cryptopithecus , Elomeryx porcinus , entelodon magnus , Eomys , Glamys fordi , Hyaenodon dubius , Isoptychus margaritae , Leptadapis sp. , Myxomygale antiqua , Paradoxonycteris tobieni , Pecora , Peratherium perriense , Pseudoltinomys gaillardi , Ronzotherium romani , Stehlinia gracilis , Tapirulus hyracinus and Tetracus .
The Hamstead Member begins at the Priabonium / Rupelium border and extends up to the Upper Rupelium. It contains the Chrons C 13n and the lower part of C 12r . The age of the member is thus between 33.75 and 32.5 million years BP .
Cranmore member
The final, 5 to 9 meter thick Cranmore member consists mainly of blue to blue-green clays. At first it comes from nor the brackish water area ( Cerithium Beds with Cerithium ), but with the nannofossils -führenden Corbula Beds (with Corbula pisum and Corbula vectensis and gastropods Hydrobia sp. , Pusillina turbinata , Sandbergeria vectiana , Strebloceras cornuides , sp Syrnola. And Teinostoma decussatum ) then clearly marine. Viviparus lentus mussels can be found on other fossils . It belongs to biostratigraphically Biozone NP23 . When the sedimentation of the Cranmore Member ended, the sea finally withdrew from the Hampshire Basin.
Sequence stratigraphic interpretation
The Bouldnor Formation belongs to two sequences of the second order, the boundary of which ( sequence boundary or SB ) runs below the Nematura Bed . The beginning of the first sequence is still at the base of the Bembridge Limestone formation. The marine intervals within the Bouldnor Formation are interpreted as high sea level . The log bed was formed during the falling stage systems tract ( FSST ), caused by the ice in the Antarctic.
The Eocene / Oligocene boundary is likely to be below the sequence boundary in the lower Hamstead member or possibly in the uppermost Bembridge-Marls member.
Note: This interpretation, advocated by JJ Hooker et al. (2009), was not accepted by AS Gale et al. (2006). They place the sequence boundary much deeper in the Bembridge Limestone Member and also divide the lower sequence into three sequences.
Grande Coupure
The Grande Coupure can be characterized using the mammalian fauna of the Bouldnor Formation as follows:
In the Upper Hamstead Member, 16 new taxa appear for the first time, 11 die out at the turn. In the Lower Hamstead member, who was before the fall of the Wall, only 5 first appearances were recorded, mainly European rodents and Butselia . Among the 16 new releases after the Grande Coupure are 10 immigrated Asian taxa. In addition, a general reduction in biodiversity can be observed. If 47 taxa were still present in the Bembridge Limestone Member, the Upper Hamstead Member can only show 28. It should be noted that the low point in species diversity was already in the Lower Hamstead Member with 20 taxa. This does not indicate sudden species extinction , but a longer process of decline beginning in the Lower Hemstead Member. The actual grand coupure of the displacement of endemic species by immigrants from Asia is likely to have happened relatively quickly.
See also
swell
literature
- Hooker, JJ: The Grande Coupure in the Hampshire Basin, UK: taxonomy and stratigraphy of the mammals on either side of this major Paleogene faunal turnover . In: Wittaker, JE & Hart, MB (Eds.): Micropalaeontology, Sedimentary Environments and Stratigraphy .
- Hooker, JJ et al. a .: Refined correlation of the UK Late Eocene-Early Oligocene Solent Group and timing of its climate history . In: Christian Koeberl & Alessandro Montanari (Eds.): The Geological Society of America Special Paper 452. The late Eocene Earth: hothouse, icehouse and impacts . 2009.
Web links
- Contribution by AS Gale et al. (PDF; 719 kB)
Individual evidence
- ↑ Insole, A. & Daley, B .: A revision of the lithostratigraphical nomenclature of the Late Eocene and Early Oligocene Strata of the Hampshire Basin, Southern England . In: Tertiary Research . tape 7 , 1985, pp. 67-100 .
- ^ Forbes, E .: On the fluvio-marine Tertiaries of the Isle of Wight . In: Quarterly Journal of the Geological Society of London . tape 9 , 1853, pp. 259-270 .
- ↑ White, HJO: A Short Account of the Geology of the Isle of Wight . In: Memoirs of the Geological Survey of Great Britain . 1921.
- ^ Paul, CRC: The molluscan faunal succession in the Hatherwood Limestone Member (Upper Eocene), Isle of Wight . In: Tertiary Research . tape 10 , 1989, pp. 147-162 .
- ^ Aubry, M.-P .: Northwestern European Palaeogene magnetostratigraphy, biostratigraphy, and paleogeography: calcareous nannofossil evidence . In: Geology . tape 13 , 1985, pp. 198-202 .
- ↑ Gale, AS et al: Correlation of Eocene-Oligocene marine and continental records: orbital cyclicity, magnetostratigraphy and sequence stratigraphy of the Solent Group, Isle of Wight, UK . In: Journal of the Geological Society . tape 163 . London 2006, p. 401-415 .